The number of patients afflicted with diabetes is as many as 100,000,000 or more throughout the world, and 90% or more thereof are afflicted with type II diabetes (Schoonjans, K. and Auwerx, J., Lancet 355: 1008-1010, 2000). Type II diabetes is characterized by impaired insulin secretion and/or insulin resistance (DeFronzo, R. A. et al., Diabetes Care 15: 318-368, 1992). Thiazolidinediones had been found both in animal experiments and in clinical studies to improve insulin resistance. When such agents were administered to patients with type II diabetes, insulin action was elevated. This adversely resulted in lowered levels of blood glucose, glycohemoglobin, and serum insulin (Schwartz, S. et al., N. Engl. J. Med. 338: 861-866, 1998).
Adiponectin, which is a plasma protein derived from the adipocyte (Maeda, K. et al., Biochem. Biophys. Res. Commun. 221: 286-289, 1996; Arita, Y. et al., Biochem. Biophys. Res. Commun. 257: 79-83, 1999) (also known as a gelatin-binding protein of 28 kDa (GBP28) (Nakano, Y. et al., J. Biochem. 120: 803-812, 1996; Saito, K. et al., Gene 229: 67-73, 1999; Saito, K. et al., Biol. Pharm. Bull. 22: 1158-1162, 1999)), adheres to a damaged vascular wall in vitro, blocks endodermal NF-κB signal transmission (Ouchi, N. et al., Circulation 102: 1296-1301, 2000), and inhibits cell proliferation of smooth muscles induced by HB-EGF and PDGF (Matsuzawa, Y. et al., Ann. N. Y. Acad. Sci. 892: 146-154, 1999). In the case of obesity patients, the levels of this protein in serum had been reported to be low (Arita, Y. et al., Biochem. Biophys. Res. Commun. 257: 79-83, 1999). In the case of patients with type II diabetes and those of coronary artery diseases, the aforementioned serum levels had been reported to be lowered (Hotta, K. et al., Arterioscler Thromb. Vasc. Biol. 20: 1595-1599, 2000).
In the past, blood glucose, HbA1C, serum insulin, HOMA-IR, and the like had been known as factors associated with type II diabetes. These factors were, however insufficient in terms of operability and/or sensitivity as indicators for diagnosing type II diabetes or for monitoring therapeutic effects. As mentioned above, association of GBP28 with type II diabetes had been also suggested. However, it was not sufficiently clarified whether or not GBP28 was actually applicable to the diagnosis of type II diabetes and the monitoring of therapeutic effects in the clinical field.
The ELISA technique of Ohmoto et al. is known as a method for assaying GBP28 (Ohmoto et al., BIO Clinica 15(10) 758-761, 2000; Japanese Laid-open Patent Publication (Kokai) No. 2000-304748). Naturally occurring GBP28 in blood is constituted by 3 monomers, and 4 to 6 trimers are aggregated (J. Biochem. 120, 803-812, 1996). However, the process of the aforementioned conventional technique was complicated due to the use of a monoclonal antibody to GBP28 having a monomeric structure. This required naturally occurring GBP28 in a sample to be denatured, a specimen to be mixed with an SDS solution at the time of assay, and thermal treatment to be conducted at 100° C. This conventional technique was to assay the naturally occurring GBP28 in a denatured state. Therefore, it could not directly assay the GBP28 in its natural state.